I tested his water, did a full water change yesterday as usual. Didn't even use vinegar!
Everything was fine! He was happy, healthy, fins un-clamped, eating, flaring, but was acting a little less active than usual. He wasn't looking too well a day ago, but I did a water change and he perked up. Today I just assumed he was having an "off" day like bettas do sometimes. I guess not...
...R.I.P Horatio.